1. Field of the Invention
This invention is broadly concerned with a so-called hybrid vehicle which has a combined power source consisting of an internal-combustion engine such as a gasoline or diesel engine and an electric motor driven by electrical energy stored in a battery, and intended particularly to be used to a hydraulic control system in the hybrid vehicle, which has a power transmitting mechanism employing a hydraulic power transmission such as a torque converter behind an output axle of the engine, and which can cool the electric motor as one of the power source of the vehicle, preferably.
2. Description of the Related Art
It is known that most of vehicles can be driven by the internal-combustion engine such as the gasoline or diesel engine in which fuel is burned. The internal-combustion engine is preferable to obtain high power to move the vehicle powerfully and to keep a long distance drive, but is inevitable to make a noise and produces exhaust gas like No.sub.x, Co.sub.2 and so on while running.
Reviewing such harm and protecting the environment, a new type vehicle using an electric motor with no noise and exhaust gas has been expected to be developed. However, a conventional electric vehicle have to mount therein a battery which is considerably heavy and of which electric capacity is limited, which is inferior to the internal-combustion engine in view of obtaining a drive power which is needed to accelerate, run under a overload state, and keep travelling at a high speed. A fatal problem in the electric vehicle is that it can not be used in various situations, since it does not enable a long cruising range on one battery charge.
It is noted that there has been invented a hybrid vehicle taking the internal-combustion engine and the electric motor together. There are two types of the hybrid vehicle in view of a combination of mentioned two power sources, one being a series-type (see, Japanese Patent Laid-open No. Sho 62-104403) wherein the vehicle is moved by a power from the electric motor which is constantly rotated by the internal-combustion engine, and the other being a parallel-type (see, Japanese Patent Laid-open No. Sho 59-63901 and U.S. Pat. No. 4,533,011) wherein the internal-combustion engine and the electric motor are parallel and selectively related with drive wheels of the vehicle.
A known cooling system for the electric motor used in the mentioned hybrid vehicles can be grouped into, for example, one of an air-cooled type, an oil-cooled type and a water-cooled type, or the other in view of a method of coolant circulation. But, the mentioned cooling system is provided only for the electric motor, which is not considered from a total point of view to effectively utilize an oil used in the power transmitting mechanism behind the engine. A plurality of axles, bearings, one-way clutches employed in the transmission do not need much oil to be lubricated for their smooth movements and cooling but have to have oil having a certain pressure. The electric motor does not need such a pressure oil but has to have much oil for its cooling.
However, as only minimum oil can be stored in a limited space, the above mentioned conventional hybrid vehicle could not control a preferable circulation of oil both for the transmission and the electric motor, so that the electric motor was not able to be used for a long term due to an insufficient oil and several axles, bearings, one-way clutches employed in the transmission were not lubricated enough due to low-pressure oil.
It is an object of the present invention to provide a hydraulic control system for the hybrid vehicle, wherein an oil used in the power transmitting mechanism behind the engine is appropriated both for cooling the electric motor with enough quantity of oil and for lubricating mechanical parts in the transmission with oil having a certain pressure.